I need to sterilise some bricks and paving stones that became infected with a toxic, spore-forming bacterium that was sold as a means of ridding a pond of duckweed (lemna), but which kills all acquatic plants. The important thing is, I need to heat the bricks and stones above 100C.

I tried the sterilisation before, but it partly failed to kill the bacterium. My guess is that this was because the objects in the oven were moist and created a lot of steam. My physics question is, do different objects inside an oven heated, say, to 250C, achieve different internal temperatures?

Put another way, if I have two identical ovens and in one place a bowl of water, is the heat of the water necessarily limited to staying below 100C?

If in the other oven I place a substance into which I can stick a thermometer, let's say sand, will that achieve 250C internal temperature?

What is the relationship between substance and achievable internal temperature? Thanks!

I tried the sterilisation before, but it partly failed to kill the bacterium. My guess is that this was because the objects in the oven were moist and created a lot of steam.

I don't know anything at all about microbiology, so I honestly don't know what would be required to make sure they're all killed off. I've been doing research into wastewater treatment plants, but most of the methods adopted at those facilities are chemical and biological, rather than physical.

Once thermal equilibrium is reached, no; the temperatures of the bricks should be the same as the oven. Note that bricks might have a large thermal mass, which means it might take a long time to reach thermal equilibrium.

Put another way, if I have two identical ovens and in one place a bowl of water, is the heat of the water necessarily limited to staying below 100C?

If the the oven is set to 250 oC, the water will turn to vapour over time and the bowl will become empty. The steam and bowl will then eventually reach 250 oC (assuming the bowl doesn't melt!).

If in the other oven I place a substance into which I can stick a thermometer, let's say sand, will that achieve 250C internal temperature?

Yes, eventually.

What is the relationship between substance and achievable internal temperature? Thanks!

All substances can reach any temperature, but it may change phase depending on the temperature and pressure of the environment (typically gas, liquid or solid). The only exception to this is that there are exotic states of matter at extremely high temperatures and pressures or extremely low temperatures and pressures (e.g. plasma, superfluids, etc.). Things get complex with these states. Fortunately, these oddities are irrelevant to your problem since you're dealing with typical substances at fairly standard temperatures and pressures.

Bricks, stone, paving slabs and other heavy construction materials tend to have very large thermal masses. Thermal mass is an engineering term which describes a substance's ability to store heat, causing a degree of "inertia" in temperature profiles throughout the material (it is also sometimes called "thermal lag"; the idea that a material can take a long time to equilibriate to changes in ambient temperature). Materials with high thermal mass have:

thanks Woody that's extremely helpful. Yes, I think that is exactly what happened. The bacterium, sold to counteract the duckweed, works by sequestering nutrients so that plants yellow and die. The stones I was using were extremely porous (limestone and brick), and the bacterium is definitely spore-forming because it was sold as a very fine white powder (which I imagine contained spores). Hence wanting to know how long it would take to generate high heat in the centres of bricks etc. It sounds as if, in a domestic oven, I might not be able to achieve enough heat to definitively destroy any spores that have penetrated and that the simplest solution is therefore to replace the edging stones for the pond with clean ones. thanks, Nigel

Hence wanting to know how long it would take to generate high heat in the centres of bricks etc. It sounds as if, in a domestic oven, I might not be able to achieve enough heat to definitively destroy any spores that have penetrated and that the simplest solution is therefore to replace the edging stones for the pond with clean ones. thanks, Nigel

100C is a very low temperature for an oven ... the problem is the bricks are porous and wet so it will take many hours to evaporate all the water

You can either leave them somewhere dry and well ventilated for a week , by which time 90% of the water will have gone , then 2 hours in the oven ...